Linux is a relatively new operating system that has begun to enjoy a lot
of attention from the business, academic and free software worlds. As the
operating system matures, its feature set, capabilities and performance
grow but so, out of necessity does its size and complexity. The table in
Figure ?? shows the size
of the kernel source code in bytes and lines of code of the mm/
part of the kernel tree. This does not include the machine dependent code
or any of the buffer management code and does not even pretend to be an
accurate metric for complexity but still serves as a small indicator.

Version

Release Date

Total Size

Size
of mm/

Line count

1.0

March 13th, 1992

5.9MiB

96KiB

3109

1.2.13

February 8th, 1995

11MiB

136KiB

4531

2.0.39

January 9th 2001

35MiB

204KiB

6792

2.2.22

September 16th, 2002

93MiB

292KiB

9554

2.4.22

August 25th, 2003

181MiB

436KiB

15724

2.6.0-test4

August 22nd, 2003

261MiB

604KiB

21714

Table 1.1: Kernel size as an indicator of complexity

As is the habit of open source developers in general, new developers
asking questions are sometimes told to refer directly to the
source with the “polite” acronym RTFS1 or else are referred to the kernel newbies mailing list
(http://www.kernelnewbies.org). With the Linux Virtual Memory
(VM) manager, this used to be a suitable response as the time required to
understand the VM could be measured in weeks and the books available devoted
enough time to the memory management chapters to make the relatively small
amount of code easy to navigate.

The books that describe the operating system such as Understanding
the Linux Kernel [BC00] [BC03],
tend to cover the entire
kernel rather than one topic with the notable exception of device
drivers [RC01]. These books, particularly Understanding
the Linux Kernel, provide invaluable insight into kernel internals but they
miss the details which are specific to the VM and not of general interest. For
example, it is detailed in this book why ZONE_NORMAL is exactly 896MiB
and exactly how per-cpu caches are implemented. Other aspects of the VM,
such as the boot memory allocator and the virtual memory filesystem which
are not of general kernel interest are also covered by this book.

Increasingly, to get a comprehensive view on how the kernel functions, one
is required to read through the source code line by line. This book tackles
the VM specifically so that this investment of time to understand it will
be measured in weeks and not months. The details which are missed by the
main part of the book will be caught by the code commentary.

In this chapter, there will be in informal introduction to the basics of
acquiring information on an open source project and some methods for managing,
browsing and comprehending the code. If you do not intend to be reading the
actual source, you may skip to Chapter 2.

One of the largest initial obstacles to understanding code is deciding
where to start and how to easily manage, browse and get an overview of the
overall code structure. If requested on mailing lists, people will provide
some suggestions on how to proceed but a comprehensive methodology is rarely
offered aside from suggestions to keep reading the source until it makes
sense. In the following sections, some useful rules of thumb for open source
code comprehension will be introduced and specifically on how they may be
applied to the kernel.

1.1.1 Configuration and Building

With any open source project, the first step is to download the source and
read the installation documentation. By convention, the source will have
a README or INSTALL file at the top-level of the
source tree [FF02]. In fact, some automated build tools such
as automake require the install file to exist. These files
will contain instructions for configuring and installing the package or
will give a reference to where more information may be found. Linux is no
exception as it includes a README which describes how the kernel
may be configured and built.

The second step is to build the software. In earlier days, the requirement
for many projects was to edit the Makefile by hand but this
is rarely the case now. Free software usually uses at leastautoconf2
to automate testing of the build environment andautomake3
to simplify the creation of Makefiles so building is often as
simple as:

mel@joshua: project $ ./configure && make

Some older projects, such as the Linux kernel, use their own configuration
tools and some large projects such as the Apache webserver have numerous
configuration options but usually the configure script is the starting
point. In the case of the kernel, the configuration is handled by the
Makefiles and supporting tools. The simplest means of configuration
is to:

mel@joshua: linux-2.4.22 $ make config

This asks a long series of questions on what type of kernel should be
built. Once all the questions have been answered, compiling the kernel
is simply:

mel@joshua: linux-2.4.22 $ make bzImage && make modules

A comprehensive guide on configuring and compiling a kernel is available with
the Kernel HOWTO4
and will not be covered in detail with this book. For now, we will presume
you have one fully built kernel and it is time to begin figuring out how
the new kernel actually works.

1.1.2 Sources of Information

Open Source projects will usually have a home page,
especially since free project hosting sites such as
http://www.sourceforge.net are available. The
home site will contain links to available documentation and instructions on
how to join the mailing list, if one is available. Some sort of documentation
will always exist, even if it is as minimal as a simple README file,
so read whatever is available. If the project is old and reasonably large, the
web site will probably feature a Frequently Asked Questions (FAQ).

Next, join the development mailing list and lurk, which means to subscribe
to a mailing list and read it without posting. Mailing lists are the
preferred form of developer communication followed by, to a lesser extent,
Internet Relay Chat (IRC) and online newgroups, commonly referred
to as UseNet. As mailing lists often contain discussions on
implementation details, it is important to read at least the previous months
archives to get a feel for the developer community and current activity. The
mailing list archives should be the first place to search if you have a
question or query on the implementation that is not covered by available
documentation. If you have a question to ask the developers, take time to
research the questions and ask it the “Right Way” [RM01]. While
there are people who will answer “obvious” questions, it will not do your
credibility any favours to be constantly asking questions that were answered
a week previously or are clearly documented.

Now, how does all this apply to Linux? First, the documentation. There is a
README at the top of the source tree and a wealth of information is
available in the Documentation/ directory. There also is a number
of books on UNIX design [Vah96], Linux specifically [BC00]
and of course this book to explain what to expect in the code.

ne of the best online sources of information
available on kernel development is the “Kernel Page” in the weekly edition
of Linux Weekly News (http://www.lwn.net). It also reports on a wide range of Linux related topics and is
worth a regular read. The kernel does not have a home web site as such but
the closest equivalent is http://www.kernelnewbies.org which is a vast source of information on the kernel that is
invaluable to new and experienced people alike.

here is a FAQ available for the Linux Kernel
Mailing List (LKML) at http://www.tux.org/lkml/ that covers
questions, ranging from the kernel development process to how to join the list
itself. The list is archived at many sites but a common choice to reference
is http://marc.theaimsgroup.com/?l=linux-kernel. Be aware that
the mailing list is very high volume list which can be a very daunting
read but a weekly summary is provided by the Kernel Traffic
site at http://kt.zork.net/kernel-traffic/.

The sites and sources mentioned so far contain general kernel information
but there are memory management specific sources. There is a Linux-MM web
site at http://www.linux-mm.org which
contains links to memory management specific documentation and a linux-mm
mailing list. The list is relatively light in comparison to the main list
and is archived at http://mail.nl.linux.org/linux-mm/.

The last site that to consult is the Kernel Trap site at
http://www.kerneltrap.org. The site contains many useful articles
on kernels in general. It is not specific to Linux but it does contain many
Linux related articles and interviews with kernel developers.

As is clear, there is a vast amount of information that is available that
may be consulted before resorting to the code. With enough experience, it
will eventually be faster to consult the source directly but when getting
started, check other sources of information first.

The mainline or stock kernel is principally distributed as a compressed tape
archive (.tar.bz) file which is available from your nearest kernel source
repository, in Ireland's case ftp://ftp.ie.kernel.org/. The stock
kernel is always considered to be the one released by the tree maintainer. For
example, at time of writing, the stock kernels for 2.2.x are those released
by Alan Cox5, for 2.4.x by Marcelo Tosatti and for 2.5.x
by Linus Torvalds. At each release, the full tar file is available as well
as a smaller patch which contains the differences between the two
releases. Patching is the preferred method of upgrading because of bandwidth
considerations. Contributions made to the kernel are almost always in the
form of patches which are unified diffs generated by the GNU
tool diff.

Why patches

Sending patches to the mailing list initially sounds clumsy but it is
remarkable efficient in the kernel development environment. The principal
advantage of patches is that it is much easier to read what changes have been
made than to compare two full versions of a file side by side. A developer
familiar with the code can easily see what impact the changes will have and
if it should be merged. In addition, it is very easy to quote the email that
includes the patch and request more information about it.

Subtrees

At various intervals, individual
influential developers may have their own version of the kernel distributed
as a large patch to the main tree. These subtrees generally contain features
or cleanups which have not been merged to the mainstream yet or are still
being tested. Two notable subtrees is the -rmap tree maintained by
Rik Van Riel, a long time influential VM developer and the -mm tree
maintained by Andrew Morton, the current maintainer of the stock development
VM. The -rmap tree contains a large set of features that for various reasons
are not available in the mainline. It is heavily influenced by the FreeBSD
VM and has a number of significant differences to the stock VM. The -mm tree
is quite different to -rmap in that it is a testing tree with patches that
are being tested before merging into the stock kernel.

BitKeeper

In more recent times, some
developers have started using a source code control system called BitKeeper
(http://www.bitmover.com), a proprietary version control system
that was designed with the Linux as the principal consideration. BitKeeper
allows developers to have their own distributed version of the tree and
other users may “pull” sets of patches called changesets from
each others trees. This distributed nature is a very important distinction
from traditional version control software which depends on a central server.

BitKeeper allows comments to be associated with each patch which is displayed
as part of the release information for each kernel. For Linux, this means
that the email that originally submitted the patch is preserved making the
progress of kernel development and the meaning of different patches a lot
more transparent. On release, a list of the patch titles from each developer
is announced as well as a detailed list of all patches included.

As BitKeeper is a proprietary product, email and patches are still considered
the only method for generating discussion on code changes. In fact,
some patches will not be considered for acceptance unless there is first
some discussion on the main mailing list as code quality is considered to
be directly related to the amount of peer review [Ray02]. As the
BitKeeper maintained source tree is exported in formats accessible to open
source tools like CVS, patches are still the preferred means of discussion. It
means that no developer is required to use BitKeeper for making contributions
to the kernel but the tool is still something that developers should be
aware of.

1.2.1 Diff and Patch

The two tools for creating and applying patches are diff and
patch, both of which are GNU utilities available from the GNU
website6. diff is used to
generate patches and patch is used to apply them. While the tools
have numerous options, there is a “preferred usage”.

Patches generated with diff should always be unified
diff, include the C function that the change affects and be generated
from one directory above the kernel source root. A unified diff include
more information that just the differences between two lines. It begins
with a two line header with the names and creation date of the two files
that diff is comparing. After that, the “diff” will consist
of one or more “hunks”. The beginning of each hunk is marked with a line
beginning with @@ which includes the starting line in the source
code and how many lines there is before and after the hunk is applied. The
hunk includes “context” lines which show lines above and below the changes
to aid a human reader. Each line begins with a +, - or
blank. If the mark is +, the line is added. If a -, the
line is removed and a blank is to leave the line alone as it is there just
to provide context. The reasoning behind generating from one directory above
the kernel root is that it is easy to see quickly what version the patch has
been applied against and it makes the scripting of applying patches easier
if each patch is generated the same way.

Let us take for example, a very simple change has been made to
mm/page_alloc.c which adds a small piece of commentary. The
patch is generated as follows. Note that this command should be all one one
line minus the backslashes.

From this patch, it is clear even at a casual glance what files are affected
(page_alloc.c), what line it starts at (76) and the new lines
added are clearly marked with a + . In a patch, there may be several “hunks”
which are marked with a line starting with @@ . Each hunk will be treated
separately during patch application.

Broadly speaking, patches come in two varieties; plain text such as the one
above which are sent to the mailing list and compressed patches that are
compressed with either gzip (.gz extension) or bzip2
(.bz2 extension). It is usually safe to assume that patches were generated
one directory above the root of the kernel source tree. This means that while
the patch is generated one directory above, it may be applied with the option
-p1 while the current directory is the kernel source tree root.
Broadly speaking, this means a plain text patch to a clean tree can
be easily applied as follows:

If a hunk can be applied but the line numbers are different, the hunk
number and the number of lines needed to offset will be output. These are
generally safe warnings and may be ignored. If there are slight differences
in the context, it will be applied and the level of “fuzziness” will be
printed which should be double checked. If a hunk fails to apply, it will
be saved to filename.c.rej and the original file will be saved
to filename.c.orig and have to be applied manually.

1.2.2 Basic Source Management with PatchSet

The untarring of sources, management of patches and building of
kernels is initially interesting but quickly palls. To cut down on
the tedium of patch management, a simple tool was developed while
writing this book called PatchSet which is designed
the easily manage the kernel source and patches eliminating a large
amount of the tedium. It is fully documented and freely available from
http://www.csn.ul.ie/∼mel/projects/patchset/ and on the
companion CD.

Downloading

Downloading kernels and patches in itself is
quite tedious and scripts are provided to make the task simpler. First,
the configuration file etc/patchset.conf should be edited
and the KERNEL_MIRROR parameter updated for your local
http://www.kernel.org/ mirror. Once that is done, use the script
download to download patches and kernel sources. A simple
use of the script is as follows

Once the relevant sources or patches have been downloaded, it is time to
configure a kernel build.

Configuring Builds

Files called set configuration files
are used to specify what kernel source tar to use, what patches to apply,
what kernel configuration (generated by make config) to use and
what the resulting kernel is to be called. A sample specification file to
build kernel 2.4.20-rmap15f is;

This first line says to unpack a source tree starting with
linux-2.4.18.tar.gz. The second line specifies that the kernel
will be called 2.4.20-rmap15f. 2.4.20 was selected
for this example as rmap patches against a later stable release were
not available at the time of writing. To check for updated rmap patches,
see http://surriel.com/patches/. The third line specifies which
kernel .config file to use for compiling the kernel. Each line
after that has two parts. The first part says what patch depth to use i.e.
what number to use with the -p switch to patch. As discussed earlier in
Section 1.2.1, this is usually 1 for applying patches
while in the source directory. The second is the name of the patch stored in
the patches directory. The above example will apply two patches to update
the kernel from 2.4.18 to 2.4.20 before building
the 2.4.20-rmap15f kernel tree.

If the kernel configuration file required is very simple, then use the
createset script to generate a set file for you. It simply takes a
kernel version as a parameter and guesses how to build it based on available
sources and patches.

mel@joshua: patchset/ $ createset 2.4.20

Building a Kernel

The package comes with three scripts. The
first script, called make-kernel.sh, will unpack the kernel
to the kernels/ directory and build it if requested. If the
target distribution is Debian, it can also create Debian packages for
easy installation by specifying the -d switch. The second,
called make-gengraph.sh, will unpack the kernel but instead of
building an installable kernel, it will generate the files required to use
CodeViz, discussed in the next section, for creating call graphs. The
last, called make-lxr.sh, will install a kernel for use with LXR.

Generating Diffs

Ultimately, you will need to see the difference
between files in two trees or generate a “diff“ of changes you have made
yourself. Three small scripts are provided to make this task easier. The
first is setclean which sets the source tree to compare from. The
second is setworking to set the path of the kernel tree you are
comparing against or working on. The third is difftree which will
generate diffs against files or directories in the two trees. To generate the
diff shown in Figure 1.2.1, the following would have worked;

The generated diff is a unified diff with the C function context included
and complies with the recommended usage of diff. Two additional
scripts are available which are very useful when tracking changes between
two trees. They are diffstruct and difffunc. These
are for printing out the differences between individual structures and
functions. When used first, the -f switch must be used to record
what source file the structure or function is declared in but it is only
needed the first time.

When code is small and manageable, it is not particularly difficult to browse
through the code as operations are clustered together in the same file and
there is not much coupling between modules. The kernel unfortunately does
not always exhibit this behaviour. Functions of interest may be spread across
multiple files or contained as inline functions in headers. To complicate
matters, files of interest may be buried beneath architecture specific
directories making tracking them down time consuming.

One solution for easy code browsing is
ctags(http://ctags.sourceforge.net/) which
generates tag files from a set of source files. These tags can be used to
jump to the C file and line where the identifier is declared with editors
such as Vi and Emacs. In the event there is multiple
instances of the same tag, such as with multiple functions with the same
name, the correct one may be selected from a list. This method works best
when one is editing the code as it allows very fast navigation through the
code to be confined to one terminal window.

A more friendly browsing method is available with the Linux
Cross-Referencing (LXR) tool hosted at http://lxr.linux.no/. This
tool provides the ability to represent source code as browsable web
pages. Identifiers such as global variables, macros and functions become
hyperlinks. When clicked, the location where it is defined is displayed
along with every file and line referencing the definition. This makes code
navigation very convenient and is almost essential when reading the code
for the first time.

The tool is very simple to install and and browsable version of the kernel
2.4.22 source is available on the CD included with this book. All
code extracts throughout the book are based on the output of LXR so that
the line numbers would be clearly visible in excerpts.

1.3.1 Analysing Code Flow

As separate modules share code across multiple C files, it can be difficult to
see what functions are affected by a given code path without tracing through
all the code manually. For a large or deep code path, this can be extremely
time consuming to answer what should be a simple question.

One simple, but effective tool to use is CodeViz
which is a call graph generator and is included with the
CD. It uses a modified compiler for either C or C++ to collect
information necessary to generate the graph. The tool is hosted at
http://www.csn.ul.ie/∼mel/projects/codeviz/.

During compilation with the modified compiler, files with a .cdep
extension are generated for each C file. This .cdep file contains
all function declarations and calls made in the C file. These files are
distilled with a program called genfull to generate a full call graph
of the entire source code which can be rendered with dot, part of the
GraphViz project hosted at http://www.graphviz.org/.

In the kernel compiled for the computer this book was written on, there
were a total of 40,165 entries in the full.graph file generated by
genfull. This call graph is essentially useless on its own because of
its size so a second tool is provided called gengraph. This program,
at basic usage, takes the name of one or more functions as an argument and
generates postscript file with the call graph of the requested function as
the root node. The postscript file may be viewed with ghostview
or gv.

The generated graphs can be to unnecessary depth or show functions that
the user is not interested in, therefore there are three limiting options
to graph generation. The first is limit by depth where functions that are
greater than N levels deep in a call chain are ignored. The second
is to totally ignore a function so it will not appear on the call graph or
any of the functions they call. The last is to display a function, but not
traverse it which is convenient when the function is covered on a separate
call graph or is a known API whose implementation is not currently of interest.

All call graphs shown in these documents are generated with the
CodeViz tool as it is often much easier to understand a subsystem
at first glance when a call graph is available. It has been tested with a
number of other open source projects based on C and has wider application
than just the kernel.

1.3.2 Simple Graph Generation

If both PatchSet and CodeViz are installed,
the first call graph in this book shown in Figure 3.4
can be generated and viewed with the following set of commands. For brevity,
the output of the commands is omitted:

When a new developer or researcher asks how to start reading the code, they
are often recommended to start with the initialisation code and work from
there. This may not be the best approach for everyone as initialisation is
quite architecture dependent and requires detailed hardware knowledge to
decipher it. It also gives very little information on how a subsystem like
the VM works as it is during the late stages of initialisation that memory
is set up in the way the running system sees it.

The best starting point to understanding the VM is this book and the code
commentary. It describes a VM that is reasonably comprehensive without being
overly complicated. Later VMs are more complex but are essentially extensions
of the one described here.

For when the code has to be approached afresh with a later VM, it is
always best to start in an isolated region that has the minimum number
of dependencies. In the case of the VM, the best starting point is the
Out Of Memory (OOM) manager in mm/oom_kill.c. It is a
very gentle introduction to one corner of the VM where a process is selected
to be killed in the event that memory in the system is low. It is because it
touches so many different aspects of the VM that is covered last in this book!
The second subsystem to then examine is the non-contiguous
memory allocator located in mm/vmalloc.c and discussed in Chapter
7 as it is reasonably contained
within one file. The third system should be physical page allocator located
in mm/page_alloc.c and discussed in Chapter
6 for similar reasons. The fourth system
of interest is the creation of VMAs and memory areas for processes discussed
in Chapter 4. Between these systems, they
have the bulk of the code patterns that are prevalent throughout the rest
of the kernel code making the deciphering of more complex systems such as
the page replacement policy or the buffer IO much easier to comprehend.

The second recommendation
that is given by experienced developers is to benchmark and test the
VM. There are many benchmark programs available but commonly used ones are
ConTest(http://members.optusnet.com.au/ckolivas/contest/),
SPEC(http://www.specbench.org/),
lmbench(http://www.bitmover.com/lmbench/ and
dbench(http://freshmeat.net/projects/dbench/). For
many purposes, these benchmarks will fit the requirements.

Unfortunately it is difficult to test just the VM accurately and
benchmarking it is frequently based on timing a task such as a kernel
compile. A tool called VM Regress is available at
http://www.csn.ul.ie/∼mel/vmregress/ that lays the foundation
required to build a fully fledged testing, regression and benchmarking tool
for the VM. It uses a combination of kernel modules and userspace tools to
test small parts of the VM in a reproducible manner and has one benchmark
for testing the page replacement policy using a large reference string. It
is intended as a framework for the development of a testing utility and has
a number of Perl libraries and helper kernel modules to do much of the work
but is still in the early stages of development so use with care.

There are two files, SubmittingPatches and CodingStyle,
in the Documentation/ directory which cover the important basics.
However, there is very little documentation describing how to get patches
merged. This section will give a brief introduction on how, broadly speaking,
patches are managed.

First and foremost, the coding style of the kernel needs to be adhered to as
having a style inconsistent with the main kernel will be a barrier to getting
merged regardless of the technical merit. Once a patch has been developed,
the first problem is to decide where to send it. Kernel development has a
definite, if non-apparent, hierarchy of who handles patches and how to get
them submitted. As an example, we'll take the case of 2.5.x development.

The first check to make is if the patch is very small or trivial. If
it is, post it to the main kernel mailing list. If there is no bad
reaction, it can be fed to what is called the Trivial Patch
Monkey7.
The trivial patch monkey is exactly what it sounds like, it takes small
patches and feeds them en-masse to the correct people. This is best suited
for documentation, commentary or one-liner patches.

Patches are managed through what could be loosely called a set of rings
with Linus in the very middle having the final say on what gets accepted
into the main tree. Linus, with rare exceptions, accepts patches only from
who he refers to as his “lieutenants”, a group of around 10 people who he
trusts to “feed” him correct code. An example lieutenant is Andrew Morton,
the VM maintainer at time of writing. Any change to the VM has to be accepted
by Andrew before it will get to Linus. These people are generally maintainers
of a particular system but sometimes will “feed” him patches from another
subsystem if they feel it is important enough.

Each of the lieutenants are active developers on different subsystems. Just
like Linus, they have a small set of developers they trust to be knowledgeable
about the patch they are sending but will also pick up patches which
affect their subsystem more readily. Depending on the subsystem, the
list of people they trust will be heavily influenced by the list of
maintainers in the MAINTAINERS file. The second major area of
influence will be from the subsystem specific mailing list if there is
one. The VM does not have a list of maintainers but it does have a mailing
list8.

The maintainers and lieutenants are crucial to the acceptance of
patches. Linus, broadly speaking, does not appear to wish to be convinced
with argument alone on the merit for a significant patch but prefers to
hear it from one of his lieutenants, which is understandable considering
the volume of patches that exists.

In summary, a new patch should be emailed to the subsystem mailing list
cc'd to the main list to generate discussion. If there is no reaction,
it should be sent to the maintainer for that area of code if there is one
and to the lieutenant if there is not. Once it has been picked up by a
maintainer or lieutenant, chances are it will be merged. The important key
is that patches and ideas must be released early and often so developers
have a chance to look at it while it is still manageable. There are notable
cases where massive patches merging with the main tree because there were
long periods of silence with little or no discussion. A recent example of
this is the Linux Kernel Crash Dump project which still has not been merged
into the main stream because there has not enough favorable feedback from
lieutenants or strong support from vendors.